1. Field of the Invention
The subject invention is generally directed to non-toxic substitutes for mercury in electrical switch and sensor applications. More particularly, the invention is directed to certain gallium alloys that have desirable properties for use in electrical switches and sensors, and to procedures and apparatuses for producing electrical switches which utilize gallium alloys.
2. Description of the Prior Art
Mercury is used extensively in switches and sensors. In a common switch application, liquid mercury is positioned inside a fluid tight housing into which a pair of spaced electrodes extend. Depending on the physical orientation of the housing, the liquid mercury can provide a conductive pathway between the electrodes or be positioned such that there is an open circuit between the electrodes. An important physical attribute of mercury is that it remains fluid throughout a wide temperature range. This attribute allows mercury to be used in many different environments and in environments with constantly changing temperature parameters. Another important physical attribute of mercury is that it has significant surface tension and does not wet glass, metal or polymer surfaces. However, mercury is toxic to humans and animals. As such, finding non-toxic alternatives to mercury that have comparable performance characteristics would be beneficial.
Two examples of prior art references which discuss gallium alloys as non-toxic substitutes for mercury in switch applications include U.S. Pat. No. 3,462,573 to Rabinowitz et al. and Japanese Patent Application Sho 57-233016 to Inage et al. Both documents identify gallium/indium/tin alloys as being potentially useful. Gallium has the advantages of remaining in the liquid phase throughout a wide temperature range and has a very low vapor pressure at atmospheric pressure. Combining other metals with gallium can depress the freezing point for the composition below that of gallium alone (29.7.degree.). Rabinowitz et al. states that a 62.5% gallium, 21.5% indium, and 16% tin composition forms an alloy that has a freezing point of 10.degree. C. The Japanese Patent Application to Inage asserts that adding 1-3.5% silver to a gallium/indium/tin alloy can lower the freezing point closer to 0.degree. C. It would be advantageous to identify an alloy which has a freezing point as close to 0.degree. C. as possible in order for the alloy to be used in the largest and broadest possible number of switch and sensor applications.
Neither Rabinowitz et al. nor Inage et al. discuss "wetting" problems encountered with gallium alloys. Rather, they suggest that the gallium alloy can be used in an envelope made of a material that is not wetted by gallium. As will be discussed below, gallium oxide, which is readily formed in gallium alloys, has the disadvantage of wetting many different surfaces.